This invention relates to optical beam delivery systems in general, and to optical beam delivery systems used with laser beams to optimize the temporal and spatial characteristics thereof.
Optical beam delivery systems are known which are used to improve the temporal and spatial characteristics of collimated beams of radiation with non-symmetrical energy profile cross sections, such as excimer laser beams. For example, in the Visx Twenty/Twenty Excimer Laser System developed by Visx Incorporated of Santa Clara, Calif., a collimated laser beam used for photorefractive keratectomy (PRK) and phototherapeutic keratectomy (PTK) is delivered to the plane of surgery by means of an optical beam delivery system which provides both spatial and temporal integration for an excimer laser beam. In this system, a collimated laser beam is first passed through a stationary spatial beam integrator comprising a plurality of prisms, which are preferably hexagonal in shape, distributed about an optical center in the form of a similar hollow space, one face of each prism being angled with respect to the central axis so that portions of a laser beam passing through each prism are refracted toward the central axis of the prism assembly. After passing through the spatial beam integrator, the laser beam is next transmitted through a temporal beam integrator comprising a dove prism which is rotated about the longitudinal optical axis in order to rotate the beam. The beam emerging from the temporal beam integrator is then directed through a variable diameter aperture and delivered to the surgical plane by means of appropriate mirrors and lenses.
While highly effective in providing spatial and temporal integration to a collimated laser beam, this arrangement is extremely sensitive to the placement of the dove prism along the optical axis of the beam delivery system. In particular, any slight misalignment of the dove prism results in a multiplication of the angular error by a factor of 2. Since any angular deviations radially displace the overlapping beam relative to the aperture, thereby affecting symmetry of the beam at the treatment site, extreme care must be taken in initially aligning the dove prism with respect to the beam axis and frequent periodic alignment checks must be made to ensure that the initial alignment has not been disturbed. Efforts to provide a spatial and temporal beam integration technique devoid of this disadvantage have not met with success to date.